The present invention relates to a method and a device for producing a tubular object from thermoplastic material, for example a bellows, the wall thickness thereof being adjustable over its length.
A method and a device of this type are, for example, known from EP 0 535 254 B1 of the applicant. The device disclosed therein comprises a die with a die land and a die body which, between them, form an annular die opening, the die body being movable relative to the die land, in order to alter the cross-sectional surface of the die opening, an extrusion device with a plunger, which during its movement extrudes a quantity of plasticized plastic material determined by the plunger path through the die, in order to produce a tubular plastic object. A withdrawal unit may be moved from an initial position adjacent to the die in the direction away from the die, in order to guide the tubular plastic object emerging from the die during the extrusion process, the path of the withdrawal unit determining the length of the tubular plastic object.
In the device disclosed therein, control devices are provided for the drives of the die body, the plunger and the withdrawal unit, which comprise a computer and path sensors for these movements, the computer controlling the three drives according to a predeterminable program. The control is carried out such that the velocity increase of the withdrawal unit is controlled during its initial movement, and in that, during the further movement of the withdrawal unit, a gap width of the die opening which is controlled so as to increase and decrease in an oscillating manner is superimposed on a continuously controlled velocity characteristic of the withdrawal unit, such that the thickness of the wall formed is altered in a wave-like manner, over valleys and peaks. The object produced in this manner is a preform for blow-molding a bellows.